Recently, the growing volume of transmission traffic has led to an increasing demand for introduction of optical transmission systems having a transmission capacity of 40 Gbit/s or more. To address this demand, various modulation schemes are being studied that are excellent in spectral efficiency, optical signal noise ratio (OSNR) tolerance, and nonlinearity tolerance compared to conventional non-return-to-zero (NRZ) modulation schemes. For example, a dual polarization-quadrature phase shift keying (DP-QPSK) modulation scheme is being studied. A digital coherent receiving method in which coherent receiving and digital signal processing are combined is also being studied. In addition, a dispersion monitor implemented in a digital coherent receiver of a digital coherent receiving scheme is known.
Note that there has been proposed a wireless communication device in which the resolution of an analog to digital converter (ADC) is switched on the basis of the reception level or the amount of change in the reception level. There has also been proposed a wireless communication device in which, on the basis of the level of a received signal (electric field intensity of a received signal), the valid bit range for a digital signal output from an ADC is limited and the signal is demodulated. There has also been proposed a communication device in which the resolution of an ADC is controlled on the basis of the transmission rate of data. There has also been proposed an orthogonal frequency division multiple (OFDM) access receiver in which the resolution of an ADC is switched on the basis of the carrier-to-noise (C/N) ratio of a received signal.
Examples of the related art include Japanese Unexamined Patent Application Publication No. 2002-368673, Japanese Unexamined Patent Application Publication No. 2001-339455, Japanese Unexamined Patent Application Publication No. 2005-159456, and Japanese Unexamined Patent Application Publication No. 2004-320377.